Conductive carbon, electrode material including said conductive carbon, and electrode using said electrode material

ABSTRACT

Provided is conductive carbon that gives an electrical storage device having a high energy density. This conductive carbon includes a hydrophilic part, and the contained amount of the hydrophilic part is 10 mass % or more of the entire conductive carbon. When performing a rolling treatment on an active material layer including an active material particle and this conductive carbon formed on a current collector during manufacture of an electrode of an electric storage device, the pressure resulting from the rolling treatment causes this conductive carbon to spread in a paste-like form and increase in density. The active material particles approach each other, and the conductive carbon is pressed into gaps formed between adjacent active material particles, filling the gaps. As a result, the amount of active material per unit volume in the electrode obtained after the rolling treatment increases, and the electrode density increases.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional application of co-pending U.S.application Ser. No. 15/123,579, filed on Sep. 2, 2016, which is theNational Phase under 35 U.S.C. § 371 of International Application No.PCT/JP2015/056562, filed on Mar. 5, 2015, which claims the benefit under35 U.S.C. § 119(a) to Patent Application No. 2014-043358, filed in Japanon Mar. 5, 2014 and Patent Application No. 2014-103849, filed in Japanon May 19, 2014, all of which are hereby expressly incorporated byreference into the present application.

TECHNICAL FIELD

The present invention relates to conductive carbon that is used in anelectric storage device with a high energy density such as a secondarybattery, an electric double layer capacitor, a redox capacitor and ahybrid capacitor. This invention also relates to an electrode materialcomprising the conductive carbon and an electrode for an electricstorage device in which this electrode material is used.

THE RELATED ART

An electric storage device such as a secondary battery, an electricdouble layer capacitor, a redox capacitor and a hybrid capacitor is adevice that is under consideration for wider application as a batteryfor an information device including a cellphone and a notebook-sizedpersonal computer, for a motor drive power supply of a low-emissionvehicle such as an electric vehicle and a hybrid vehicle, and for anenergy recovery system, etc. In these devices, improvement in energydensity is desired to meet the requirements of higher performance anddownsizing.

In these electric storage devices, an electrode active material thatrealizes its capacity by a faradaic reaction involving the transfer ofan electron with an ion in an electrolyte (including an electrolyticsolution) or by a nonfaradaic reaction not involving the transfer of anelectron is used for energy storage. Further, this active material isgenerally used in the form of a composite material with anelectroconductive agent. As the electroconductive agent, conductivecarbon such as carbon black, natural graphite, artificial graphite, andcarbon nanotube is generally used. This conductive carbon, usedconcurrently with a low conductive active material, serves to addconductivity to the composite material, and furthermore, acts as amatrix to absorb the volume change in accordance with the reaction ofthe active material. Also, it serves to ensure an electron conductingpath when the active material is mechanically damaged.

The composite material of the active material and the conductive carbonis generally manufactured by a method of mixing particles of the activematerial and the conductive carbon. The conductive carbon does not makea significant contribution to the improvement of the energy density ofan electric storage device, so the quantity of the conductive carbon perunit volume needs to be decreased and that of the active material needsto be increased to obtain an electric storage device with a high energydensity. Therefore, consideration is given to a method to decrease thedistance between the particles of the active material to increase thequantity of the active material per unit volume by improving thedispersibility of the conductive carbon or by reducing the structure ofthe conductive carbon.

For example, Patent Document 1 (JP 2004-134304 A) discloses a nonaqueoussecondary battery that is equipped with a positive electrode thatcontains a small-sized carbon material having an average primaryparticle diameter of 10 to 100 nm (in its example, acetylene black) andthat has a degree of blackness of 1.20 or more. A coating material usedto form the positive electrode is obtained either by dispersing amixture of an active material for a positive electrode, theabovementioned carbon material, a binder and a solvent by a high sheardispersing machine such as a high speed rotational homogenizerdispersing machine or a planetary mixer with three or more rotary axes,or by adding a dispersion body, in which a mixture of the abovementionedcarbon material, a binder and a solvent are dispersed by a high sheardispersing machine, into a paste in which a mixture of the activematerial for a positive electrode, a binder and a solvent are dispersed,and further dispersing. By using the device that has a high shearingforce, the carbon material, which is hard to disperse because of itssmall particle size, becomes evenly dispersed.

Also, Patent Document 2 (JP 2009-35598 A) discloses an electroconductiveagent for an electrode for a nonaqueous secondary battery that consistsof acetylene black whose BET-specific surface area is 30 to 90 m²/g,dibutylphthalate (DBP) oil absorption quantity is 50 to 120 mL/100 g,and pH is 9 or more. The electrode for the secondary battery is formedby dispersing a mixture of this acetylene black and an active materialin a fluid containing a binder to prepare slurry, and applying thisslurry on a current collector and drying it. Since the acetylene blackwith the abovementioned characteristics has a smaller structure comparedwith Ketjen Black or other conventional acetylene blacks, the bulkdensity of a mixture of the acetylene black and the active material isimproved and the battery capacity is improved.

PRIOR ARTS DOCUMENTS Patent Documents

Patent Document 1: JP 2004-134304 A

Patent Document 2: JP 2009-35598 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Further improvement of an electric storage device in terms of energydensity is always desired. However, the inventors have examined theprior arts and found that even by the methods disclosed in PatentDocuments 1 and 2, it is difficult to enable conductive carbon toinfiltrate efficiently between particles of an active material, andtherefore, it is difficult to shorten the distance between the activematerial particles and increase the amount of the active material perunit volume. Therefore, the inventors have found that there is alimitation to the improvement of the energy density with a positiveelectrode and/or a negative electrode using the composite material ofparticles of an active material and conductive carbon.

Therefore, the objective of the present invention is to provideconductive carbon that gives an electric storage device with a highenergy density.

Means for Solving Problems

After a keen examination, the inventors have found that electrodedensity significantly increases by forming an electrode of an electricstorage device by using a composite material of conductive carbon, whichis obtained by giving a strong oxidizing treatment to a raw material ofconductive carbon, and a particle of an active material. Moreover,extensive analysis of the conductive carbon used has revealed that aconductive carbon gives an electric storage device with a high energydensity if the conductive carbon comprises a lot of hydrophilic part andthe contained amount of the hydrophilic part is 10% by mass or more ofthe entire conductive carbon.

Therefore, the present invention, first of all, relates to conductivecarbon for an electrode of an electric storage device, wherein theconductive carbon comprises a hydrophilic part, and the contained amountof the hydrophilic part is 10% by mass or more of the entire conductivecarbon.

In the present invention, the “hydrophilic part” of conductive carbonmeans the following: 0.1 g of conductive carbon is added to 20 ml of anammonia aqueous solution with pH 11, ultrasonic irradiation is appliedfor 1 minute, and a fluid obtained is left for 5 hours to precipitateits solid phase part. The part that does not precipitate and isdispersed in the ammonia aqueous solution with pH 11 is the “hydrophilicpart.” Also, the contained amount of the hydrophilic part in the entireconductive carbon can be calculated by the following method: After theprecipitation of the solid phase part, the supernatant fluid is removed,the remaining part is dried, and the weight of the solid object afterdrying is measured. The weight calculated by subtracting the weight ofthe solid object after drying from the weight of the initial conductivecarbon (0.1 g) is the weight of the “hydrophilic part,” which isdispersed in the ammonia aqueous solution with pH 11. The weight ratioof the weight of the “hydrophilic part” against the weight of theinitial conductive carbon (0.1 g) is the contained amount of the“hydrophilic part” in the conductive carbon.

The ratio of the hydrophilic part in conductive carbon such as carbonblack, natural graphite and carbon nanotube, which is used as aconductive agent in an electrode of a conventional electric storagedevice, is 5% by mass or less of the entire conductive carbon. However,by using these conductive carbons as raw materials and giving anoxidizing treatment to these raw materials, the surface of theirparticles is oxidized and a hydroxy group, a carboxy group and an etherbond are introduced into the carbon, and a conjugated double bond of thecarbon is oxidized so that a carbon single bond is formed, acarbon-carbon bond is partially severed, and a hydrophilic part isformed on the surface of the particles. Then, as the intensity of theoxidizing treatment is increased, the percentage of the hydrophilic partin the carbon particle is increased and the hydrophilic part accountsfor 10% by mass or more of the entire conductive carbon, and particlesaggregate by the intermediary of this hydrophilic part.

FIG. 1 is a model drawing to show the change in such an aggregated bodywhen pressure is applied to the aggregated body. As pressure is appliedto the aggregated body aggregated by the intermediary of the hydrophilicpart as shown in the upper drawing of FIG. 1, a vulnerable hydrophilicpart becomes unable to bear the pressure and the aggregated bodycollapses as shown in the middle drawing of FIG. 1. As pressure isfurther increased, not only the hydrophilic part on the surface ofcarbon particles, but also a non-oxidized part at the center of theparticles is transformed or partially collapses and the entire aggregatebody becomes compressed. However, the hydrophilic part of the carbonparticles serves as a binder even when the aggregate body is compressed,and as shown in the lower drawing of FIG. 1 the non-oxidized part andthe hydrophilic part of the carbon particles become unified and spreadin a paste-like manner. Then the carbon that spreads in a paste-likemanner is compressed extremely densely. The word “paste-like” refers toa condition in which the grain boundary of carbon primary particles isnot observed and non-particulate amorphous carbons are connected witheach other in an SEM image at a magnification of 25,000.

The conductive carbon of the present invention is characterized aseasily attachable to the surface of the particles of the active materialand it is compressed integrally and spreads in a paste-like manner whenpressure is applied to it, and is hard to separate. Therefore, when theconductive carbon of the present invention and the particles of anactive material are mixed and an electrode material is obtained for anelectrode of an electric storage device, the conductive carbon isattached to and covers the surface of the particles of the activematerial in the process of mixing and the dispersity of the particles ofthe active material is improved. When the pressure applied to theconductive carbon in manufacturing the electrode material is large, atleast part of the conductive carbon spreads in a paste-like manner andthe surface of the particles of the active material becomes partiallycovered. Moreover, if an active material layer is formed with thiselectrode material on a current collector of an electrode and pressureis applied to the active material layer, as shown in the lower drawingof FIG. 1 most or all of the conductive carbon of the present inventionspreads in a paste-like manner and becomes dense while covering thesurface of the particles of the active material, the particles of theactive material approach each other, and accordingly the conductivecarbon of the present invention is pushed out not only into the gap thatis formed between the adjacent particles of the active material, butalso into pores that exist on the surface of the particles of the activematerial (including gaps between primary particles that are found insecondary particles) and fill the gaps and pores while covering thesurface of the particles of the active material (see FIG. 3). Therefore,the amount of the active material per unit volume in the electrode isincreased and electrode density is increased. Moreover, the paste-likeconductive carbon that is densely filled has sufficient conductivity toserve as a conductive agent and does not inhibit the impregnation of anelectrolytic solution in the electric storage device. As a result, theenergy density of the electric storage device is improved.

On the other hand, when conductive carbon such as carbon black that isused as a conductive agent in an electrode of a conventional electricstorage device, or an oxide of such carbon that does not have sufficientoxidizing intensity, in which the contained amount of the hydrophilicpart is less than 10% by mass of the entire conductive carbon, is usedto compose an electrode of an electric storage device, the increase inelectrode density is not sufficient. In the conductive carbon of thepresent invention, it is preferable that the contained amount of thehydrophilic part is 12% by mass or more and 30% by mass or less so thatan especially high electrode density can be obtained.

The conductive carbon of the present invention can be suitablymanufactured by a oxidizing treatment of a carbon raw material having aninner vacancy. The inner vacancy includes a pore in porous carbon powderas well as a hollow of Ketjen Black, an internal or interstitial pore ofa carbon nanofiber or a carbon nanotube.

As mentioned above, when a composite material of the conductive carbonof the present invention and particles of an electrode active materialis employed as an electrode material to form an electrode of an electricstorage device, the energy density of the electric storage device isimproved. Therefore, the present invention also relates to an electrodematerial that is an electrode material for an electric storage deviceand comprises the conductive carbon of this invention and an electrodeactive material particle.

In the electrode material of the present invention, it is preferablethat an average diameter of the electrode active material particles iswithin a range of 0.01 to 2 μm. Since the conductive carbon of thepresent invention attaches to and covers the surface of the particles ofthe active material, aggregation of the active material particles can beinhibited even if the average diameter of the particles of the activematerial is as small as 0.01 to 2 μm.

In the electrode material of the present invention, it is preferablethat the electrode active material particles are composed of fineparticles with an average diameter of 0.01 to 2 μm that are operable asa positive electrode active material or a negative electrode activematerial and gross particles with an average diameter of more than 2 μmand not more than 25 μm that are operable as an active material of thesame electrode as the fine particles. The gross particles increase theelectrode density and improve the energy density of an electric storagedevice. Also, due to the pressure applied to the electrode material inmanufacturing the electrode, the fine particles press the conductivecarbon of the present invention, and are pushed out into and fill thegaps that are formed between adjacent gross particles together with theconductive carbon, so that the electrode density further increases andthe energy density of the electric storage device further improves. Theaverage diameter of the active material particles is the 50% radius(median diameter) as in the measurement of particle size distributionobtained by using a light scattering particle size meter.

In the electrode material of the present invention, it is preferablethat another kind of conductive carbon, especially conductive carbonthat has higher electroconductivity than the conductive carbon of thepresent invention, is further comprised. When pressure is applied to theelectrode material when the electrode is manufactured, this carbon alsodensely fills the gaps formed by the adjacent particles of an activematerial together with the conductive carbon of the present inventionand the conductivity of the whole electrode is improved, so that theenergy density of an electric storage device further improves.

As mentioned above, the energy density of an electric storage device isimproved when the electrode of the electric storage device is composedwith the electrode material that comprises the conductive carbon of thepresent invention and an active material particle. Therefore, thepresent invention also relates to an electrode for an electric storagedevice which comprises an active material layer formed by addingpressure to the electrode material of the present invention. Also, ithas been found that the dissolution of the active material by anelectrolytic solution in the electric storage device is significantlyinhibited and the cycle characteristic of the electric storage device issignificantly improved, conceivably because most or all of the surfaceof the particles of the active material in the electrode, including theinside of the pores that exist on the surface of the particles of theactive material, is covered by the conductive carbon of the presentinvention that spreads in a paste-like manner.

Advantageous Effects of the Invention

The conductive carbon of the present invention attaches easily to thesurface of the particles of the active material and it is compressedintegrally and spreads in a paste-like manner when pressure is appliedto it, and is hard to separate, so that the conductive carbon isattached to and covers the surface of the particles of an activematerial in the process of mixing the conductive carbon of the presentinvention and the particles of the active material and obtaining theelectrode material, and the dispersity of the particles of the activematerial is improved. Also, in manufacturing the electrode of theelectric device, when pressure is applied to the electrode materialcomprising the particles of an active material and the conductive carbonof the present invention, due to the pressure most or all of theconductive carbon of the present invention spreads in a paste-likemanner and becomes dense while covering the surface of the particles ofthe active material, the particles of the active material approach eachother, and accordingly the conductive carbon of the present invention ispushed out not only into the gap that is formed between the adjacentparticles of the active material, but also into pores that exist on thesurface of the particles of the active material and fill the gaps andpores densely. Therefore, the amount of the active material per unitvolume in the electrode is increased and electrode density is increased.Moreover, the paste-like conductive carbon that is densely filled hassufficient conductivity to serve as a conductive agent and does notinhibit the impregnation of an electrolytic solution in the electricstorage device. As a result, the energy density of the electric storagedevice is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a model drawing that shows the change in an aggregated bodyof conductive carbon of a working example when pressure is applied tothe aggregated body.

FIG. 2 shows a graph in which the relationship between the containedamount of a hydrophilic part and the electrode density is shown forconductive carbons of working examples and comparative examples.

FIG. 3 shows SEM images of the cross-section of an electrode with anactive material layer that comprises conductive carbon of a workingexample and particles of an active material: (A) at a magnification of1,500 and (B) at a magnification of 25,000.

FIG. 4 shows SEM images of the cross-section of an electrode with anactive material layer that comprises conductive carbon of a comparativeexample and particles of an active material: (A) at a magnification of1,500 and (B) at a magnification of 25,000.

FIG. 5 shows a graph that shows the result of measuring the distributionof pores in electrodes shown in FIGS. 3 and 4 by the nitrogen gasadsorption method.

FIG. 6 shows the rate characteristics of a lithium ion secondary batterythat has an electrode with an active material layer that comprisesconductive carbon of a working example or a comparative example andparticles of an active material.

FIG. 7 shows a graph that shows the cycle characteristics of a lithiumion secondary battery, the rate characteristics of which are shown inFIG. 6.

FIG. 8 shows the rate characteristics of a lithium ion secondary batterythat has an electrode with an active material layer that comprisesconductive carbon of a working example or a comparative example andparticles of another kind of active material.

FIG. 9 shows a graph that shows the cycle characteristics of a lithiumion secondary battery, the rate characteristics of which are shown inFIG. 8.

FIG. 10 shows the rate characteristics of a lithium ion secondarybattery that has an electrode with an active material layer thatcomprises conductive carbon of a working example or a comparativeexample and particles of another kind of active material.

FIG. 11 shows a graph that shows the cycle characteristics of a lithiumion secondary battery, the rate characteristics of which are shown inFIG. 10.

FIG. 12 shows an SEM image at a magnification of 50,000 of a mixturethat is obtained by dry blending of conductive carbon of a workingexample, acetylene black and particles of an active material.

FIG. 13 shows the rate characteristics of a lithium ion secondarybattery that has an electrode with an active material layer thatcomprises conductive carbon of a working example or a comparativeexample and particles of an active material.

FIG. 14 shows a graph that shows the cycle characteristics of a lithiumion secondary battery, the rate characteristics of which are shown inFIG. 13.

FIG. 15 shows SEM images of conductive carbons of a working example anda comparative example.

FIG. 16 shows graphs that show the result of surveving the influence ofthe pressure on conductive carbons of a working example and acomparative example: (A) shows the relationship between the pressure anddensity and (B) shows the relationship between the pressure and theincreased rated of density.

FIG. 17 shows graphs that show the result of surveying the influence ofthe pressure on conductive carbons of a working example and acomparative example: (A) shows the relationship between the pressure andelectroconductivity and (B) shows the relationship between the pressureand the increased rate of electroconductivity.

FIG. 18 shows SEM images at a magnification of 50,000 of a mixture thatis obtained by dry blending of conductive carbon of a working example oracetylene black and fine particles of an active material.

FIG. 19 shows SEM images at a magnification of 100,000 of a mixture thatis obtained by dry blending of conductive carbon of a working example oracetylene black and gross particles of an active material.

DETAILED DESCRIPTION OF THE INVENTION

The conductive carbon of the present invention has a hydrophilic partand the contained amount of the hydrophilic part is 10% by mass or more,preferably 12% by mass or more and 30% by mass or less, of the entireconductive carbon. When an oxidizing treatment is applied to a carbonraw material, preferably a carbon raw material with an inner vacancy,the surface of its particles is oxidized, and a hydroxy group, a carboxygroup and an ether bond are introduced into the carbon, and a conjugateddouble bond of the carbon is oxidized, so that a carbon single bond isformed, a carbon-carbon bond is partially severed, and a hydrophilicpart is formed on the surface of the particles. Then, as the intensityof the oxidizing treatment is increased, the ratio of the hydrophilicpart in the carbon particle is increased and the hydrophilic partaccounts for 10% by mass or more of the entire conductive carbon.

Such conductive carbon easily attaches to the surface of the particlesof the active material and it is compressed integrally and spreads in apaste-like manner when pressure is applied to it, and is hard toseparate.

The conductive carbon of the present invention can be suitably obtainedby the first manufacturing method comprising:

(a1) a process in which acidic treatment is given to a carbon rawmaterial with an inner vacancy;

(b1) a process in which the product after acidic treatment and atransition metal compound are mixed;

(c1) a process in which the mixture obtained is pulverized to produce amechanochemical reaction;

(d1) a process in which the product after the mechanochemical reactionis heated in a nonoxidizing atmosphere; and

(e1) a process in which the aforementioned transition metal compoundand/or its reaction product is removed from the product after heating.

In the first manufacturing method, carbon with an inner vacancy such asporous carbon powder, Ketjen Black furnace black with pores, carbonnanofiber and carbon nanotube is used as the carbon raw material. Assuch a carbon raw material with an inner vacancy, the use of carbon inwhich the specific surface area of micropores, which have a diameter of2 nm or less as measured by the MP method, is 200 m²/g or more ispreferable, the use of carbon in which the average primary particlediameter is 200 nm or less is preferable, and the use of carbon in whichthe shape of the particles are spherical is preferable. The carbon rawmaterial within these ranges is easily denatured into the conductivecarbon of the present invention by the first manufacturing method.Especially, spherical particles such as Ketjen Black and furnace blackwith pores are preferable. It is difficult to obtain the conductivecarbon of the present invention by using solid carbon as a raw materialand applying the same treatment as the first manufacturing method to thesolid carbon.

In the (a1) process, the carbon raw material is left immersed in acid.An ultrasonic wave can be irradiated during this immersion. As acid, anacid usually used for an oxidizing treatment of carbon such as nitricacid, a mixture of nitric acid and sulfuric acid, and an aqueoussolution of hypochlorous acid can be used. The immersion time depends onthe concentration of acid or the quantity of the carbon raw material tobe treated, and is usually within the range of 5 minutes to 5 hour. Thecarbon after acidic treatment is sufficiently washed by water and dried,and then mixed with a transition metal compound in the (b1) process.

For the chemical compound of transition metal to be added to the carbonraw material in the (b1) process, an inorganic metallic salt oftransition metal such as a halide, nitrate, sulfate and carbonate; anorganic metallic salt of transition metal such as formate, acetate,oxalate, methoxide, ethoxide and isopropoxide; or a mixture thereof canbe used. These chemical compounds can be used alone, or two or morekinds can be used as a mixture. Chemical compounds that containdifferent transition metals can be mixed in a prescribed amount andused. Also, a chemical compound other than the chemical compound oftransition metal, such as an alkali metal compound, can be addedconcurrently unless it has an adverse effect on the reaction. Since theconductive carbon of the present invention is mixed with particles of anactive material and used in manufacturing an electrode of an electricstorage device, it is preferable that a chemical compound of an elementconstituting the active material is added to the carbon raw material sothat adulteration of an element that can serve as impurities against theactive material can be prevented.

In the (c1) process, the mixture obtained in the (b1) process ispulverized and a mechanochemical reaction is produced. Examples of apowdering machine for this reaction are a mashing machine, ball mill,bead mill, rod mill, roller mill, agitation mill, planetary mill,vibrating mill, hybridizer, mechanochemical composite device and jetmill. Milling time depends on the powdering machine used or the quantityof the carbon to be treated and has no strict restrictions, but isgenerally within the range of 5 minutes to 3 hours. The (d1) process isconducted in a nonoxidizing atmosphere such as a nitrogen atmosphere andan argon atmosphere. The temperature and time of heating is chosen inaccordance with the chemical compound of transition metal used. In thesubsequent (e1) process, the conductive carbon of the present inventioncan be obtained by removing the chemical compound of transition metaland/or its reaction product from the product that has been heated bymeans of acid dissolution etc., then sufficiently washing and drying.

In the first manufacturing method, the chemical compound of transitionmetal promotes the oxidation of the carbon raw material by themechanochemical reaction in the (c1) process, and the oxidation of thecarbon raw material rapidly proceeds. By this oxidation, the conductivecarbon that comprises a hydrophilic part, which is 10% by mass or moreof the entire conductive carbon, can be obtained.

The conductive carbon of the present invention can also be suitablyobtained by the second manufacturing method that comprises:

(a2) a process in which a carbon raw material with an inner vacancy anda chemical compound of transition metal are mixed;

(b2) a process in which the mixture obtained is heated in an oxidizingatmosphere; and

(c2) a process in which the abovementioned chemical compound oftransition metal and/or its reaction product is removed from the productafter heat treatment.

In the second manufacturing method, as the carbon raw material, carbonwith an inner vacancy such as porous carbon powder, Ketjen Black,furnace black with pores, carbon nanofiber and carbon nanotube is used.As a carbon raw material with an inner vacancy, the use of carbon inwhich the specific surface area of micropores, which have a diameter of2 nm or less as measured by the MP method, is 200 m²/g or more, ispreferable, the use of carbon in which the average primary particlediameter is 200 nm or less is preferable, and the use of carbon in whichthe shape of the particles is spherical is preferable. The carbon rawmaterial within these ranges is easily denatured into the conductivecarbon of the present invention by the second manufacturing method.Especially, spherical particles such as Ketjen Black and furnace blackwith pores are preferable. It is difficult to obtain the conductivecarbon of the present invention by using solid carbon as a raw materialand giving the same treatment as the second manufacturing method to thesolid carbon.

As the chemical compound of transition metal to be added to the carbonraw material in the (a2) process, an inorganic metallic salt oftransition metal such as a halide, nitrate, sulfate and carbonate; anorganic metallic salt of transition metal such as formate, acetate,oxalate, methoxide, ethoxide and isopropoxide; or a mixture thereof canbe used. These chemical compounds can be used alone, or two or morekinds can be mixed and used. Chemical compounds that contain differenttransition metals can be mixed in a prescribed amount and used.Moreover, a chemical compound other than a chemical compound oftransition metal such as a chemical compound of alkali metal can beadded concurrently unless it has an adverse effect on the reaction. Thisconductive carbon is mixed with particles of an active material and usedin manufacturing an electrode of an electric storage device, so it ispreferable to add a chemical compound of an element that constitutes theactive material to the carbon raw material because this will prevent themixing of an element that can be impurities against the active material.

The (b2) process is conducted in an oxidizing atmosphere, for example inair, and at a temperature at which carbon partially disappears but notcompletely disappears, preferably at a temperature of 200 to 350° C. Inthe subsequent (c2) process, the conductive carbon of the presentinvention can be obtained by removing the chemical compound oftransition metal and/or its reaction product from the product that hasbeen heated by means of acid dissolution etc., then sufficiently washingand drying.

In the second manufacturing method, the chemical compound of transitionmetal acts as a catalyst to oxidize the carbon raw material in theheating process in an oxidizing atmosphere and the oxidation of thecarbon raw material rapidly proceeds. By this oxidation, the conductivecarbon that comprises a hydrophilic part, which is 10% by mass or moreof the entire conductive carbon, can be obtained.

The conductive carbon of the present invention can be obtained by givinga strong oxidizing treatment to a carbon raw material, but it is alsopossible to promote the oxidation of the carbon raw material by a methodother than the first manufacturing method or the second manufacturingmethod.

The conductive carbon of the present invention is used for an electrodeof an electric storage device such as a secondary battery, an electricdouble layer capacitor, a redox capacitor and a hybrid capacitor in anembodiment in which the conductive carbon of the present invention ismixed with a particle of an electrode active material that realizes itscapacity by a faradaic reaction that involves the transfer of anelectron between an ion in an electrolyte of the electric storage deviceor a nonfaradaic reaction that does not involve the transfer of anelectron. The electric storage device comprises a pair of electrodes (apositive electrode and a negative electrode) and an electrolyte that isplaced between the electrodes as essential elements, and at least one ofthe positive electrode and the negative electrode is manufactured withan electrode material comprising the conductive carbon of the presentinvention and an electrode active material particle.

The electrolyte that is placed between a positive electrode and anegative electrode in an electric storage device can be an electrolyticsolution that is held by a separator, a solid electrolyte, or a gelelectrolyte, that is, an electrolyte that is used in a conventionalelectric storage device can be used without any restrictions.Representative electrolytes are as follows. For a lithium ion secondarybattery, an electrolytic solution in which a lithium salt such as LiPF₆,LiBF₄, LiCF₃SO₃ and LiN(CF₃SO₂)₂ is dissolved in a solvent such asethylene carbonate, propylene carbonate, butylene carbonate anddimethylcarbonate can be used and held by a separator such as polyolefinfiber nonwoven fabric and glass fiber nonwoven fabric. Further, aninorganic solid electrolyte such as Li₅La₃Nb₂O₁₂,Li_(1.5)Al_(0.5)Ti_(1.5)(PO₄)₃, Li₇La₃Zr₂O₁₂ and Li₇P₃S₁₁, an organicsolid electrolyte that is composed of a complex of a lithium salt and amacromolecule compound such as polyethylene oxide, polymethacrylate andpolyacrylate, and a gel electrolyte in which an electrolytic solution isabsorbed into polyvinylidene fluoride and polyacrylonitrile etc. arealso used. For an electric double layer capacitor and a redox capacitor,an electrolytic solution in which a quaternary ammonium salt such as(C₂H₅)₄NBF₄ is dissolved in a solvent such as acrylonitrile andpropylene carbonate is used. For a hybrid capacitor, an electrolyticsolution in which a lithium salt is dissolved in propylene carbonateetc. or an electrolytic solution in which a quaternary ammonium salt isdissolved into propylene carbonate etc. is used.

The positive electrode or negative electrode of an electric storagedevice is generally manufactured by sufficiently kneading an electrodematerial comprising the conductive carbon of the present invention andthe particles of an electrode active material together with a solvent inwhich a binder is dissolved as needed, forming an active material layerby applying the kneaded material obtained onto a current collector toform the positive electrode or negative electrode of the electricstorage device, drying this active material layer as needed, and thengiving the active material layer a rolling treatment. In the case wherea solid electrolyte or a gel electrolyte is used as an electrolytebetween a positive electrode and a negative electrode, a solidelectrolyte is added to an electrode material comprising the conductivecarbon of the present invention and an electrode active materialparticle in order to ensure an ion conductive pass in the activematerial layer. The mixture obtained is sufficiently kneaded togetherwith a solvent in which a binder is dissolved as needed, an activematerial layer is formed using the kneaded material obtained.

In the process of manufacturing an electrode material by mixing theconductive carbon of the present invention and the particles of anelectrode active material, aggregation of the particles of an activematerial can be inhibited because the conductive carbon is attached toand covers the surface of the particles of the active material. If thepressure applied to the conductive carbon in manufacturing the electrodematerial is large, at least part of the conductive carbon spreads in apaste-like manner and the surface of the particles of an electrodeactive material is partially covered. Also, by the pressure appliedduring a rolling treatment to an active material layer, most or all ofthe conductive carbon of the present invention spreads in a paste-likemanner and becomes dense while covering the surface of the particles ofthe active material, the particles of the active material approach eachother, and accordingly the conductive carbon of the present invention ispushed out not only into the gaps formed between the adjacent particlesof the active material, but also into the pores that exist on thesurface of the particles of the active material, and fills the gaps andpores while covering the surface of the particles of the activematerial. Therefore, the amount of the active material per unit volumein the electrode is increased and electrode density is increased.Moreover, the paste-like conductive carbon that is densely filled hassufficient conductivity to serve as a conductive agent and does notinhibit the impregnation of an electrolytic solution in the electricstorage device. As a result, the energy density of the electric storagedevice is improved.

As the active material for a positive electrode and an active materialfor a negative electrode that are mixed with the conductive carbon ofthe present invention in the manufacture of an electrode material, anactive material for an electrode that is used in a conventional electricstorage device can be used without any specific restrictions. The activematerial can be a single chemical compound or a mixture of two or morekinds of chemical compound.

Examples of a positive electrode active material for a secondary batteryare, among all, LiMO₂ having a laminar rock salt structure, laminarLi₂MnO₃—LiMO₂ solid solution, and spinel LiM₂O₄(M in the formulasignifies Mn, Fe, Co, Ni or a combination thereof). Specific examples ofthese are LiCoO₂, LiNiO₂, LiNi_(4/5)Co_(1/5)O₂,LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, LiNi_(1/2)Mn_(1/2)O₂, LiFeO₂, LiMnO₂,Li₂MnO₃—LiCoO₂, Li₂MnO₃—LiNiO₂, Li₂MnO₃—LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂,Li₂MnO₃—LiNi_(1/2)Mn_(1/2)O₂,Li₂MnO₃—LiNi_(1/2)Mn_(1/2)O₂—LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, LiMn₂O₄ andLiMn_(3/2)Ni_(1/2)O₄. Other examples include sulfur and a sulfide suchas Li₂S, TiS₂, MoS₂, FeS₂, VS₂ and Cr_(1/2)V_(1/2)S₂, a selenide such asNbSe₃, VSe₂ and NbSe₃, an oxide such as Cr₂O₅, Cr₃O₈, VO₂, V₃O₈, V₂O₅and V₆O₁₃ as well as a complex oxide such asLiNi_(0.8)Co_(0.15)Al_(0.05)O₂, LiVOPO₄, LiV₃O₅, LiV₃O₈, MoV₂O₈,Li₂FeSiO₄, Li₂MnSiO₄, LiFePO₄, LiFe_(1/2)Mn_(1/2)PO₄, LiMnPO₄ andLi₃V₂(PO₄)₃.

Examples of a negative electrode active material for a secondary batteryare an oxide such as Fe₂O₃, MnO, MnO₂, Mn₂O₃, Mn₃O₄, CoO, Co₃O₄, NiO,Ni₂O₃, TiO, TiO₂, SnO, SnO₂, SiO₂, RuO₂, WO, WO₂ and ZnO, metal such asSn, Si, Al and Zn, a complex oxide such as LiVO₂, Li₃VO₄ and Li₄Ti₅O₁₂,and a nitride such as Li_(2.6)Co_(0.4)N, Ge₃N₄, Zn₃N₂ and Cu₃N.

As an active material in a polarizable electrode of an electric doublelayer capacitor, a carbon material with a large specific surface areasuch as activated carbon, carbon nanofiber, carbon nanotube, phenolresin carbide, polyvinylidene chloride carbide and microcrystal carbonis exemplified. In a hybrid capacitor, a positive electrode activematerial exemplified for a secondary battery can be used as a positiveelectrode. In this case, a negative electrode is composed of apolarizable electrode using activated carbon etc. Also, a negativeelectrode active material exemplified for a secondary battery can beused as a negative electrode. In this case, a positive electrode iscomposed of a polarizable electrode using activated carbon etc. As apositive electrode active material of a redox capacitor, a metal oxidesuch as RuO₂, MnO₂ and NiO is exemplified, and a negative electrode iscomposed of an active material such as RuO₂ and a polarizable materialsuch as activated carbon.

There is no limitation to the shape or particle diameter of theparticles of an active material, but it is preferable that the averageparticle diameter of the particles of the active material is within therange of 0.01 to 2 μm. The particles of the active material that have anaverage particle diameter within the range of 0.01 to 2 μm are easy toaggregate, but in the process to obtain an electrode material by mixingthe particles of the active material with the conductive carbon of thepresent invention, the conductive carbon is attached to and covers thesurface of the particles of the active material, so that aggregation ofthe particles of the active material can be inhibited even if theaverage particle diameter of the particles of the active material issmall, and the mixing state of the particles of the active material andthe conductive carbon can be uniformalized. Also, it is preferable thatthe average particle diameter of the particles of the active material ismore than 2 μm and not more than 25 μm. The particles of the activematerial that have such a relatively large diameter improve electrodedensity, and in the process of mixing in manufacturing the electrodematerial, gelatinization of the conductive carbon is promoted by thecompressing strength of the particles. Also, in the process of applyingpressure to the active material layer on the current collector inmanufacturing the electrode, the particles of an active material thathave such a relatively large particle diameter further press theconductive carbon, at least a part of which is gelatinized, and make thecarbon further spread in a paste-like manner, and make the carbondenser. As a result, electrode density further increases and the energydensity of an electric storage device further improves.

Also, it is preferable that the particles of an active material arecomposed of fine particles with an average diameter of 0.01 to 2 μm andgross particles with an average diameter of more than 2 μm and not morethan 25 μm that are operable as an active material of the same electrodeas the fine particles. In the process of manufacturing an electrodematerial, the conductive carbon of the present invention is attached toand covers the surface not only of the fine particles, but also of thegross particles, so that aggregation of the particles of the activematerial can be inhibited, and the mixing state of the particles of theactive material and the conductive carbon can be uniformalized. Also, asmentioned above, the gross particles promote gelatinization anddensification of the conductive carbon of the present invention,increase electrode density, and improve the energy density of theelectric storage device. Further, by the pressure applied by a rollingtreatment to an active material layer that is formed on a currentcollector in manufacturing an electrode, the fine particles press theconductive carbon of the present invention and are pushed out into andfill the gaps that are formed between adjacent gross particles togetherwith the conductive carbon which is spread in a paste-like manner, sothat the electrode density further increases and the energy density ofthe electric storage device further improves.

Also, the conductive carbon of the present invention can be usedconcurrently with conductive carbon other than the conductive carbon ofthe present invention, including carbon black such as Ketjen Black,acetylene black, furnace black and channel black, fullerene, carbonnanotube, carbon nanofiber, graphene, amorphous carbon, carbon fiber,natural graphite, artificial graphite, graphitized Ketjen Black,mesoporous carbon, and vapor grown carbon fiber etc., which is used foran electrode of a conventional electric storage device. Especially, itis preferable to use concurrently carbon that has a higherelectroconductivity than the electroconductivity of the conductivecarbon of the present invention. Since the conductive carbon of thepresent invention is attached to and covers not only the surface of theactive material particles, but also the surface of the conductive carbonused concurrently, the aggregation of the conductive carbon usedconcurrently can be inhibited. Moreover, by the pressure added to theactive material layer formed on the current collector by a rollingtreatment in manufacturing the electrode, the conductive carbon usedconcurrently densely fills the gap formed between the adjacent particlestogether with the conductive carbon of the present invention that isspread in a paste-like manner, and the electroconductivity of the entireelectrode improves, and thus the energy density of the electric storagedevice further improves.

The method to mix the active material particles, the conductive carbonof the present invention and the other conductive carbon usedconcurrently as needed has no restrictions, and a heretofore knownmethod of mixing can be used. However, it is preferable to mix by drymixing, and for dry mixing a mashing machine, ball mill, bead mill, rodmill, roller mill, agitation mill, planetary mill, vibration mill,hybridizer, mechanochemical composite device and jet mill can be used.Especially, it is preferable to give a mechanochemical treatment to theactive material particles and the conductive carbon of the presentinvention because the coatability and the evenness of the covering ofthe active material particles by the conductive carbon of the presentinvention are improved. The ratio of the amount of the active materialparticles and that of the conductive carbon of the present invention orthe total amount of the conductive carbon of the present invention andthe other conductive carbon used concurrently as needed is preferablywithin the range of 90:10 to 99.5:0.5 mass ratio and more preferablywithin the range of 95:5 to 99:1 in order to obtain an electric storagedevice with a high energy density. If the ratio of the conductive carbonis lower than the abovementioned range, the conductivity of the activematerial layer tends to become insufficient, and the covering rate ofthe active material particles by the conductive carbon tends todecrease. Also, if the ratio of the conductive carbon is larger than theabovementioned range, the electrode density tends to decrease and theenergy density of the electric storage device tends to decrease.

As the current collector for an electrode of an electric storage device,an electroconductive material such as platinum, gold, nickel, aluminum,titanium, steel and carbon can be used. For the form of the currentcollector, any form such as a film, foil, plate, net, expanded metal, orcylinder can be adopted.

As the binder to be mixed with the electrode material, a heretoforeknown binder such as polytetrafluoroethylene, polyvinylidene fluoride,tetrafluoroethylene-hexafluoropropylene copolymer, polyvinyl fluorideand carboxymethylcellulose can be used. It is preferable that the amountof binder used is 1 to 30% by mass of the total amount of the mixedmaterial. If the amount of binder used is 1% by mass or less, thestrength of the active layer is not sufficient, and if the amount ofbinder used is 30% by mass or more, drawbacks such as a decrease in thedischarge capacity of an electrode or excessive internal resistancearise. As the solvent to be mixed with the electrode material, a solventsuch as N-methyl pyrrolidone that does not adversely affect theelectrode material can be used without any restriction.

By measuring the pore distribution of the active material layer in theelectrode that has the active material layer comprising the conductivecarbon of the present invention by using the nitrogen gas adsorptionmethod, it was found the active material layer has pores with a diameterof 5 to 40 nm. These tiny pores are considered to be pores within thedense, paste-like conductive carbon and these pores are of sufficientlylarge a size to allow an electrolytic solution in an electric storagedevice to go through the paste-like conductive carbon to the activematerial particles. Therefore, the paste-like conductive carbon in theactive material layer does not inhibit the impregnation of theelectrolytic solution in the electric storage device. Moreover, it wasfound that the dense, paste-like conductive carbon also exists in thegaps formed between the adjacent particles of the electrode activematerial and/or inside the pores that exist on the surface of theelectrode active material, the gaps and the pores having a width of 50nm or less. Therefore, the conductive property of the entire activematerial layer is improved and the electrode density is also improved.Conductive carbon such as carbon black, natural graphite and carbonnanotube, which are used as a conductive agent in an electrode of aconventional electric storage device, can hardly intrude into gaps orpores of such narrow width. The term “the width of the gaps formedbetween the adjacent particles of the electrode active material” meansthe shortest distance of the distances between adjacent particles of theelectrode active material and the term “the width of the pores thatexist on the surface of the electrode active material” means theshortest distance of the distances between the points at the oppositeends of the pores.

In the electrode that has the active material layer comprising theconductive carbon of the present invention, most or all of the surfaceof the particles of the active material in the active material layer arecovered by the conductive carbon of the present invention that is denseand spreads in a paste-like manner to the inside of the pores that existon the surface of the active material particle, and 80% or more of thesurface (outer surface) of the particles of the active material,preferably 90% or more, and especially preferably 95% or more contactsthe dense, paste-like conductive carbon. Probably because of this,dissolution of the active material in the electrolytic solution of theelectric storage device is significantly inhibited if the electricstorage device is composed of this electrode. Here, the solutionquantity of the active material decreases by approximately 40% or morecompared the case where an electrode is composed of a conventionalconductive agent such as acetylene black and an active materialparticle. Moreover, due to the fact that solution of the active materialis significantly inhibited, the cycling characteristics of the electricstorage device are significantly improved. The coverage rate of thesurface of the particles of the active material by the paste-likeconductive carbon is a value calculated by observation of SEM images ofthe cross-sectional surface of the active material layer at amagnification of 25,000.

EXAMPLES

The present invention is explained in the following examples, though thepresent invention is not limited to the following examples.

(1) The First Manufacturing Method, the Evaluation of the ContainedAmount of the Hydrophilic Part and Electrode Density

Example 1

Ketjen Black (trade name: EC300J, manufacturer: Ketjen BlackInternational Co., with average primary particle diameter: 40 nm, thespecific surface area of micropores with a diameter of 2 nm or less,derived from the measurement result by the MP method using the nitrogenadsorption method: 430 m²g⁻¹) weighing 10 g was added to 300 mL of 60%nitric acid and then the fluid obtained was irradiated by an ultrasonicwave for 10 minutes, and then the fluid was filtered and the KetjenBlack was retrieved. The retrieved Ketjen Black was washed with waterthree times and then dried, so that oxidized Ketjen Black was obtained.Then, 0.5 g of the oxidized Ketjen Black obtained was mixed with 1.98 gFe(CH₃COO)₂, 0.77 g Li(CH₃COO), 1.10 g C₆H₈O₇.H₂O, 1.32 g CH₃COOH, 1.31g H₃PO₄, and 120 mL distilled water, and the mixed fluid obtained wasagitated by a stirrer for 1 hour, and then the mixed fluid wasevaporated, dried and solidified at 100° C. in air and a mixture wascollected. Then, the mixture obtained was introduced into a vibratoryball mill device and pulverization was conducted at 20 hz for 10minutes. The powder obtained by pulverization was heated at 700° C. for3 minutes in nitrogen, and a complex in which LiFePO₄ was supported byKetjen Black was obtained.

1 g of the complex obtained was added to 100 mL of 30% hydrochloric acidaqueous solution, then the LiFePO₄ in the complex was dissolved byirradiating the fluid obtained with an ultrasonic wave for 15 minutes,and the remaining solid matter was filtered, washed with water anddried. A part of the solid matter after drying was heated to 900° C. inair and its weight loss was measured by TG analysis. Until it wasconfirmed that the weight loss was 100%, that is, no LiFePO₄ remained,the abovementioned process of dissolving LiFePO₄ in the hydrochloricacid aqueous solution, filtering, washing with water and drying wasrepeated, so that conductive carbon that did not contain any LiFePO₄ wasobtained.

Then, 0.1 g of the conductive carbon obtained was added to 20 ml ofammonia solution with pH 11, and ultrasonic irradiation was conductedfor 1 minute. The fluid obtained was left for 5 hours and a solid phasearea was precipitated. After the precipitation of the solid phase area,the supernatant fluid was removed, the remaining part was dried, and theweight of the solid object after drying was measured. By subtracting theweight of the solid object after drying from the weight of the initialconductive carbon (0.1 g) and calculating the weight ratio of thesubtracted result against the initial weight of the conductive carbon(0.1 g), the contained amount of the “hydrophilic part” in theconductive carbon was evaluated.

Fe(CH₃COO)₂, Li(CH₃COO), C₆H₈O₇.H₂O, CH₃COOH and H₃PO₄ were introducedinto distilled water, and the compound fluid obtained was agitated by astirrer for 1 hour, and then the compound fluid was evaporated, driedand solidified at 100° C. in air and then heated at 700° C. for 3minutes in nitrogen, and LiFePO₄ fine particles with an initial particlediameter of 100 nm (the average particle diameter: 100 nm) wereobtained. Then, commercially available LiFePO₄ gross particles (initialparticle diameter: 0.5 to 1 μm, secondary particle diameter: 2 to 3 μm,the average particle diameter: 2.5 μm), the fine particles obtained andthe abovementioned conductive carbon were mixed at the ratio of 90:9:1,and an electrode material was obtained. Then, 5% by mass of the totalmass of polyvinylidene fluoride and an adequate quantity of N-methylpyrrolidone were added to the electrode material and kneadedsufficiently so that slurry was formed, and this slurry was coated on analuminum foil, dried and then given a rolling treatment, and anelectrode with an active material layer was obtained. The electrodedensity of the electrode was calculated from the measured values of thevolume and weight of the active material layer on the aluminum foil inthe electrode.

Example 2

The procedure of Example 1 was repeated except that the process in which0.5 g oxidized Ketjen Black, 1.98 g Fe(CH₃COO)₂, 0.77 g Li(CH₃COO), 1.10g C₆H₈O₇.H₂O, 1.32 g CH₃COOH, 1.31 g H₃PO₄ and 120 mL distilled waterwere mixed was changed into a process in which 1.8 g oxidized KetjenBlack, 0.5 g Fe(CH₃COO)₂, 0.19 g Li(CH₃COO), 0.28 g C₆H₈O₇.H₂O, 0.33 gCH₃COOH, 0.33 g H₃PO₄ and 250 mL distilled water were mixed.

Example 3

10 g of Ketjen Black used in Example 1 was added to 300 mL of 40% nitricacid and then the fluid obtained was irradiated by an ultrasonic wavefor 10 minutes, and then the fluid was filtered and the Ketjen Black wasretrieved. The retrieved Ketjen Black was washed with water three timesand then dried, so that oxidized Ketjen Black was obtained. Then, theprocedure of Example 2 was repeated except that this oxidized KetjenBlack 1.8 g was used instead of the oxidized Ketjen Black 1.8 g used inExample 2.

Comparative Example 1

10 g of Ketjen Black used in Example 1 was added to 300 mL of 60% nitricacid and then the fluid obtained was irradiated by an ultrasonic wavefor 1 hour, and then the fluid was filtered and the Ketjen Black wasretrieved. The retrieved Ketjen Black was washed with water three timesand then dried, so that oxidized Ketjen Black was obtained. Thisoxidized Ketjen Black was heated at 700° C. for 3 minutes in nitrogen.For the conductive carbon obtained, the contained amount of thehydrophilic part was measured by using the same procedure as theprocedure in Example 1. Also, an electrode containing LiFePO₄ wasproduced by using the same procedure as the procedure in Example 1 andits electrode density was calculated.

Comparative Example 2

10 g of Ketjen Black used in Example 1 was added to 300 mL of 30% nitricacid and then the fluid obtained was irradiated by an ultrasonic wavefor 10 minutes, and then the fluid was filtered and the Ketjen Black wasretrieved. The retrieved Ketjen Black was washed with water three timesand then dried, so that oxidized Ketjen Black was obtained. Then,without pulverization by vibratory ball mill, it was heated at 700° C.for 3 minutes in nitrogen. For the conductive carbon obtained, thecontained amount of the hydrophilic part was measured by using the sameprocedure as the procedure in Example 1. Also, an electrode containingLiFePO₄ was produced by using the same procedure as the procedure inExample 1 and its electrode density was calculated.

Comparative Example 3

To confirm the contribution of the hydrophilic part to electrodedensity, 40 mg of the conductive carbon of Example 1 was added to 40 mLof pure water, ultrasonic irradiation was applied for 30 minutes todisperse the carbon in the pure water, then the dispersion was left for30 minutes, after which the supernatant fluid was removed, then theremaining part was dried, and a solid object was obtained. For thissolid object, the contained amount of the hydrophilic part was measuredby using the same procedure as the procedure in Example 1. Also, usingthe conductive carbon obtained, an electrode containing LiFePO₄ wasproduced by using the same procedure as the procedure in Example 1 andits electrode density was calculated.

Comparative Example 4

For the Ketjen Black raw material used in Example 1, the containedamount of the hydrophilic part was measured by using the same procedureas the procedure in Example 1. Also, using the conductive carbon, anelectrode containing LiFePO₄ was produced by using the same procedure asthe procedure in Example 1 and its electrode density was calculated.

FIG. 2 is a graph that shows the relationship between the containedamounts of the hydrophilic part in the conductive carbons of Examples 1to 3 and Comparative Examples 1 to 4 and the electrode densities ofelectrodes of Examples 1 to 3 and Comparative Examples 1 to 4. As isevident from FIG. 2, if the contained amount of the hydrophilic partexceeds 8% by mass of the entire conductive carbon, the electrodedensity begins to increase, and if it exceeds 9% by mass of the entireconductive carbon, the electrode density begins to increase sharply, andif the contained amount of the hydrophilic part exceeds 10% by mass ofthe entire conductive carbon, the high electrode density of 2.6 g/cc ormore can be obtained. Also, as is evident from the comparison of theresult for Example 1 and the result for Comparative Example 3, thehydrophilic part of the conductive carbon largely contributes to theimprovement of electrode density.

(2) The Second Manufacturing Method, the Evaluation of the ContainedAmount of the Hydrophilic Part and Electrode Density

Example 4

Ketjen Black (trade name: EC300J, manufacturer: Ketjen BlackInternational Co., with average primary particle diameter: 40 nm, thespecific surface area of micropores: 430 m²g⁻¹) weighing 0.45 g wasmixed with 4.98 g Co(CH₃COO)₂.4H₂O, 1.6 g LiOH.H₂O and 120 mL distilledwater, and the mixed fluid obtained was agitated by a stirrer for 1hour, and then a mixture was collected by filtering. Then, 1.5 gLiOH.H₂O was mixed through an evaporator and then heated at 250° C. for30 minutes in air, and a complex in which a lithium cobalt chemicalcompound was supported by Ketjen Black was obtained. 1 g of the complexobtained was added to 100 mL aqueous solution in which concentratedsulfuric acid (98%), concentrated nitric acid (70%) and hydrochloricacid (30%) were mixed at the volume ratio of 1:1:1, and the lithiumcobalt chemical compound in the complex was dissolved by irradiating themixed fluid obtained with an ultrasonic wave for 15 minutes, and thenthe residual solid matter was filtered, washed with water and dried. Apart of the solid matter after drying was heated to 900° C. in air andits weight loss was measured by TG analysis. Until it was confirmed thatthe weight loss was 100%, that is, no lithium cobalt chemical compoundremained, the abovementioned process of dissolving the lithium cobaltchemical compound in the abovementioned acid aqueous solution,filtering, washing with water and drying was repeated, so thatconductive carbon that did not contain any lithium cobalt chemicalcompound was obtained.

Then, for the conductive carbon obtained, the contained amount of thehydrophilic part was measured by using the same procedure as theprocedure in Example 1. The contained amount of the hydrophilic part was14.5% by mass of the entire carbon.

Li₂CO₃, Co(CH₃COO)₂ and C₆H₈O₇.H₂O were introduced into distilled water,the compound fluid obtained was agitated by a stirrer for 1 hour, andthen the compound fluid was evaporated, dried and solidified at 100° C.in air and was then heated at 800° C. in air for 10 minutes, and LiCoO₂fine particles with an average diameter of 0.5 μm were obtained. Then,commercially available LiCoO₂ gross particles (average diameter: 10 μm),the fine particles obtained and the abovementioned conductive carbonwere mixed at the mass ratio of 90:9:1, and 5% by mass of the total massof polyvinylidene fluoride and an adequate quantity of N-methylpyrrolidone were added and kneaded sufficiently so that slurry wasformed, and this slurry was coated on an aluminum foil, dried and thengiven a rolling treatment, and an electrode with an active materiallayer was obtained. The electrode density of the electrode wascalculated from the measured values of the volume and weight of theactive material layer on the aluminum foil in the electrode. The valueof the electrode density was 4.2 g/cc.

Comparative Example 5

For the Ketjen Black raw material used in Example 4, the containedamount of the hydrophilic part was measured by using the same procedureas the procedure in Example 1. The contained amount of the hydrophilicpart was 5% by mass of the entire carbon. Also, an electrode containingLiCoO₂ was produced by using the same procedure as the procedure inExample 4 and its electrode density was calculated. The value of theelectrode density was 3.6 g/cc.

Comparison of Example 4 and Comparative Example 5 revealed that theelectrode density significantly improves if the contained amount of thehydrophilic part is 10% by mass or more of the entire conductive carbon.

(3) Evaluation as a Lithium Ion Secondary Battery

(i) Active Material: LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂

Example 5

Li₂CO₃, Ni(CH₃COO)₂, Mn(CH₃COO)₂ and Co(CH₃COO)₂ were introduced intodistilled water, the compound fluid obtained was agitated by a stirrerfor 1 hour, and then the compound fluid was evaporated, dried andsolidified at 100° C. in air, mixed by using a ball mill, and thenheated at 800° C. in air for 10 minutes, andLiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ fine particles with an average diameter of0.5 μm were obtained. These fine particles and the conductive carbon inExample 1 were mixed at the ratio by mass of 90:10 and a preliminarycompound was obtained. Then, 86% by mass of commercially availableLiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ gross particles (average diameter: 5 μm),9% by mass of the abovementioned preliminary compound and 2% by mass ofacetylene black (primary particle diameter: 40 nm, specific surface areaof micropores: 0 m²g⁻¹) were mixed, and then 3% by mass ofpolyvinylidene fluoride and an adequate quantity of N-methyl pyrrolidonewere added and kneaded sufficiently so that slurry was formed, and thisslurry was coated on an aluminum foil, dried and then given a rollingtreatment, and a positive electrode with an active material layer for alithium ion secondary battery was obtained. The electrode density of thepositive electrode was calculated from the measured values of the volumeand weight of the active material layer on the aluminum foil in thepositive electrode. The value of the electrode density was 4.00 g/cc.Further, using the positive electrode obtained, a lithium ion secondarybattery was manufactured in which 1M LiPF₆ in a 1:1 ethylenecarbonate/diethyl carbonate solution was used as an electrolyticsolution, and in which lithium was used as a counter electrode.Charging/discharging characteristics of the lithium ion secondarybattery obtained were measured for a broad range of current densities.

Example 6

94% by mass of commercially available LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂particles (average diameter: 5 μm), 2% by mass of the conductive carbonof Example 1 and 2% by mass of acetylene black (primary particlediameter: 40 nm, specific surface area of micropores: 0 m²g⁻¹) weremixed, and then 2% by mass of polyvinylidene fluoride and an adequatequantity of N-methyl pyrrolidone were added and kneaded sufficiently sothat slurry was formed, and this slurry was coated on an aluminum foil,dried and then given a rolling treatment, and a positive electrode withan active material layer for a lithium ion secondary battery wasobtained. The electrode density of the positive electrode was calculatedfrom the measured values of the volume and weight of the active materiallayer on the aluminum foil in the positive electrode. The value of theelectrode density was 3.81 g/cc. Further, using the positive electrodeobtained, a lithium ion secondary battery was manufactured in which 1MLiPF₆ in a 1:1 ethylene carbonate/diethyl carbonate solution was used asan electrolytic solution, and in which lithium was used as a counterelectrode. Charging/discharging characteristics of the lithium ionsecondary battery obtained were measured for a broad range of currentdensities.

Comparative Example 6

94% by mass of commercially available LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂particles (average diameter: 5 μm) and 4% by mass of acetylene black(primary particle diameter: 40 nm, specific surface area of micropores:0 m²g⁻¹) were mixed, and then 2% by mass of polyvinylidene fluoride andan adequate quantity of N-methyl pyrrolidone were added and kneadedsufficiently so that slurry was formed, and this slurry was coated on analuminum foil, dried and then given a rolling treatment, and a positiveelectrode with an active material layer for a lithium ion secondarybattery was obtained. The electrode density of the positive electrodewas calculated from the measured values of the volume and weight of theactive material layer on the aluminum foil in the positive electrode.The value of the electrode density was 3.40 g/cc. Further, using thepositive electrode obtained, a lithium ion secondary battery wasmanufactured in which 1M LiPF₆ in a 1:1 ethylene carbonate/diethylcarbonate solution was used as an electrolytic solution, and in whichlithium was used as a counter electrode. Charging/dischargingcharacteristics of the lithium ion secondary battery obtained weremeasured for a broad range of current densities.

FIG. 3 shows SEM images of the cross-section of the positive electrodeof the lithium ion secondary battery of Example 6 and FIG. 4 shows SEMimages of the cross-section of the positive electrode of the lithium ionsecondary battery of Comparative Example 6. In both figures, (A) is animage at a magnification of 1,500 and (B) is an image at a magnificationof 25,000. In FIG. 3(A) and FIG. 4(A), the thickness of the activematerial layer is shown as t. It can be seen that the active materiallayer in the lithium ion secondary battery of Example 6 is thinner thanthe active material layer in the lithium ion secondary battery ofComparative Example 6, even though the contained amount of the particlesof the active material and the contained amount of carbon in the activematerial layer are the same. Also, from the comparison of FIG. 3(A) andFIG. 4(A), it was found that, in the active material layer of thelithium ion secondary battery of Example 6, the particles of the activematerial approach each other and the ratio of the area occupied bycarbon to the area of the entire active material layer in the image issmall. Further, the forms of carbon in FIG. 3(B) and FIG. 4(B) areremarkably different. In the active material layer of the lithium ionsecondary battery of Comparative Example 6 (FIG. 4(B)), the grainboundaries of carbon (acetylene black) primary particles are clear andthere are large gaps adjacent to the surface boundary between theparticles of the active material and the carbon particles, especiallyadjacent to the pores formed on the surface of the particles of theactive material, in addition to the gaps between the carbon particles,whereas in the active material layer of the lithium ion secondarybattery of Example 6 (FIG. 3(B)), the grain boundaries of carbon primaryparticles are not discernible, the carbon is paste-like and thispaste-like carbon intrudes into the deep parts of the pores of theparticles of the active material that have a width of 50 nm or less (ie.gaps between the primary particles), and gaps are virtually absent.Moreover, it is shown that 90% or more of the surface of the particlesof the active material contact the paste-like carbon. It is assured thatthe difference between the electrode densities in the positiveelectrodes of the lithium ion secondary batteries in Example 6 andComparative Example 6 is derived from the above-described difference inthe forms of carbon.

As mentioned above, because the active material layer in Example 6 isthinner than the active material layer in Comparative Example 6, it isassured that the material filling rate in the former is large, and thematerial filling rate was confirmed in the following formulae. Thetheoretical electrode density refers to the electrode density when gapsin the active material layer are assumed to be 0%.Material filling rate (%)=electrode density×100/theoretical electrodedensity  (I)Theoretical electrode density (g/cc)=100/{a/X+b/Y+(100−a−b)/Z}  (II)

where:

a: % by mass of the active material against the entire active materiallayer

b: % by mass of carbon against the entire active material layer

100−a−b: % by mass of polyvinylidene fluoride against the entire activematerial layer

X: true density of the active material

Y: true density of carbon black

Z: true density of polyvinylidene fluoride

As a result, the material filling rate of the active material layer inExample 6 was 86.8% and the material filling rate of the active materiallayer in Comparative Example 6 was 79.1%; in the electrode containingthe conductive carbon of the present invention, an improvement in thefilling rate of as much as 7.7% was observed.

FIG. 5 shows the result of the measurement of the pore distribution inthe active material layer of Example 6 and the active material layer ofComparative Example 6 by the nitrogen gas adsorption method. The resultshows that, in the active material layer in Comparative Example 6, poreswith a diameter of 20 nm or less are virtually absent, and most of thepores show peaks at the diameter of approximately 30 nm, the diameter ofapproximately 40 nm, and the diameter of approximately 150 nm.Presumably, the pores that show a peak at the diameter of approximately150 nm are pores that are mainly attributable to the particles of anactive material and the pores that show peaks at the diameter ofapproximately 30 nm and the diameter of approximately 40 nm are thepores that are mainly found between particles of acetylene black. On theother hand, it is assured that, in the active material layer in Example6, the number of pores with a diameter of approximately 100 nm or moreamong the pores in the active material layer in Comparative Example 6 isdecreased, and instead the number of pores with a diameter within therange of 5 to 40 nm is increased. It is considered that the decrease inthe number of pores with a diameter of approximately 100 nm or more isbecause the pores of the particles of the active material are coveredwith the paste-like conductive carbon. Moreover, the pores with adiameter within the range of 5 to 40 nm, which are presumably pores inthe dense, paste-like conductive carbon, are of a sufficiently largesize to allow for the electrolytic solution in an electric storagedevice to go through the paste-like conductive carbon to contact theactive material particles. Therefore, it is concluded that thepaste-like conductive carbon in the electrode does not inhibit theimpregnation of the electrolytic solution in the electric storagedevice.

FIG. 6 is a graph that shows the relationship between the rate and thedischarge capacity per volume of the positive electrode active materiallayer in the lithium ion secondary batteries of Example 5, Example 6 andComparative Example 6. The lithium ion secondary battery of Example 6shows a higher capacity than the lithium ion secondary battery ofComparative Example 6, and the lithium ion secondary battery of Example5 shows a higher capacity than the lithium ion secondary battery ofExample 6. That is, as the electrode density of the positive electrodeincreases, the discharge capacity per volume also increases. Also, thesesecondary batteries show almost the same rate characteristics. Thisreveals that the paste-like conductive carbons contained in the activematerial layers in the secondary batteries of Example 5 and Example 6have sufficient electroconductivity to serve as a conductive agent anddo not inhibit the impregnation of the electrolytic solution in asecondary battery. Also, the positive electrode of the secondary batteryin Example 5 shows a higher electrode density than the positiveelectrode of the secondary battery in Example 6, even though thecontained amount of the particles of the active material and thecontained amount of carbon in the active material layer are almost thesame, which is presumably because the fine particles are pushed out intoand fill the gaps formed between the adjacent gross particles togetherwith the conductive carbon of Example 1 while pressing the conductivecarbon.

For the lithium secondary batteries of Example 6 and Comparative Example6, charging/discharging was repeated within the range of 4.6 to 3.0 Vunder the condition of 60° C. and the charging/discharging rate of 0.5C. FIG. 7 shows the result of the cycling characteristics obtained. Theresult shows that the secondary battery of Example 6 has better cyclecharacteristics than the secondary battery of Comparative Example 6.From a comparison of FIG. 3 and FIG. 4, it is considered that this isbecause almost all of the surface of the particles of the activematerial in the active material layer in Example 6 is covered withpaste-like carbon up to the point of the depth of the pores on thesurface of the particles of the active material and that this paste-likecarbon inhibits the degradation of the active material.

(ii) Active Material: LiCoO₂

Example 7

Li₂CO₃, Co(CH₃COO)₂ and C₆H₈O₇.H₂O were introduced into distilled water,the compound fluid obtained was agitated by a stirrer for 1 hour, andthen the compound fluid was evaporated, dried and solidified at 100° C.in air and was then heated at 800° C. in air for 10 minutes, and LiCoO₂fine particles with an average diameter of 0.5 μm were obtained. Theseparticles and the conductive carbon obtained in Example 1 were mixed atthe mass ratio of 90:10, and a preliminary mixture was obtained. Then,86% by mass of the total mass of commercially available LiCoO₂ grossparticles (average particle diameter: 10 μm), 9% by mass of theabovementioned preliminary mixture and 2% by mass of acetylene black(primary particle diameter: 40 nm, specific surface area of micropores:0 m²g⁻¹) were mixed, and then 3% by mass of polyvinylidene fluoride andan adequate quantity of N-methyl pyrrolidone were added and kneadedsufficiently so that slurry was formed, and this slurry was coated on analuminum foil, dried and then given a rolling treatment, and a positiveelectrode with an active material layer for a lithium ion secondarybattery was obtained. The electrode density of the positive electrodewas calculated from the measured values of the volume and weight of theactive material layer on the aluminum foil in the positive electrode.The value of the electrode density was 4.25 g/cc. Further, using thepositive electrode obtained, a lithium ion secondary battery wasmanufactured in which 1M LiPF₆ in a 1:1 ethylene carbonate/diethylcarbonate solution was used as an electrolytic solution, and in whichlithium was used as a counter electrode. Charging/dischargingcharacteristics of the lithium ion secondary battery obtained weremeasured for a broad range of current densities.

Example 8

94% by mass of commercially available LiCoO₂ particles (averagediameter: 10 μm), 2% by mass of the conductive carbon of Example 1 and2% by mass of acetylene black (primary particle diameter: 40 nm,specific surface area of micropores: 0 m²g⁻¹) were mixed, and then 2% bymass of polyvinylidene fluoride and an adequate quantity of N-methylpyrrolidone were added and kneaded sufficiently so that slurry wasformed, and this slurry was coated on an aluminum foil, dried and thengiven a rolling treatment, and a positive electrode with an activematerial layer for a lithium ion secondary battery was obtained. Theelectrode density of the positive electrode was calculated from themeasured values of the volume and weight of the active material layer onthe aluminum foil in the positive electrode. The value of the electrodedensity was 4.05 g/cc. Further, using the positive electrode obtained, alithium ion secondary battery was manufactured in which 1M LiPF₆ in a1:1 ethylene carbonate/diethyl carbonate solution was used as anelectrolytic solution, and in which lithium was used as a counterelectrode. Charging/discharging characteristics of the lithium ionsecondary battery obtained were measured for a broad range of currentdensities.

Comparative Example 7

94% by mass of commercially available LiCoO₂ particles (averagediameter: 10 μm) and 4% by mass of acetylene black (primary particlediameter: 40 nm, specific surface area of micropores: 0 m²g⁻¹) weremixed, and then 2% by mass of polyvinylidene fluoride and an adequatequantity of N-methyl pyrrolidone were added and kneaded sufficiently sothat slurry was formed, and this slurry was coated on an aluminum foil,dried and then given a rolling treatment, and a positive electrode withan active material layer for a lithium ion secondary battery wasobtained. The electrode density of the positive electrode was calculatedfrom the measured values of the volume and weight of the active materiallayer on the aluminum foil in the positive electrode. The value of theelectrode density was 3.60 g/cc. Further, using the positive electrodeobtained, a lithium ion secondary battery was manufactured in which 1MLiPF₆ in a 1:1 ethylene carbonate/diethyl carbonate solution was used asan electrolytic solution, and in which lithium was used as a counterelectrode. Charging/discharging characteristics of the lithium ionsecondary battery obtained were measured for a broad range of currentdensities.

For the active material layer in Example 8 and the active material layerin Comparative Example 7, the material filling rates were confirmed byusing the abovementioned formulae (I) and (II). As a result, thematerial filling rate of the active material layer in Example 8 was85.6% and the material filling rate of the active material layer inComparative Example 7 was 79.1%; in the electrode containing theconductive carbon of the present invention, an improvement in thefilling rate of as much as 6.5% was observed.

FIG. 8 is a graph that shows the relationship between the rate and thedischarge capacity per volume of the positive electrode active materiallayer in the lithium ion secondary batteries of Example 7, Example 8 andComparative Example 7. In line with the result shown in FIG. 6, FIG. 8shows that the discharging capacity increases as the electrode densityincreases and almost the same rate characteristics are obtained. For thelithium ion secondary batteries of Example 8 and Comparative Example 7,charging/discharging was repeated within the range of 4.3 to 3.0 V underthe condition of 60° C. and the charging/discharging rate of 0.5 C. FIG.9 shows the result of the cycling characteristics obtained. In line withthe result shown in FIG. 7, FIG. 9 shows that the secondary battery ofExample 8 has better cycle characteristics than the secondary battery ofComparative Example 7.

(iii) Active Material: Li_(1.2)Mn_(0.56)Ni_(0.17)Co_(0.07)O₂

Example 9

1.66 g Li(CH₃COO), 2.75 g Mn(CH₃COO)₂.4H₂O, 0.85 g Ni(CH₃COO)₂.4H₂O,0.35 g Co(CH₃COO)₂.4H₂O and 200 mL distilled water were mixed, solventwas removed by using the evaporator, and a mixture was collected. Then,the mixture collected was introduced into a vibratory ball mill device,pulverization at 15 hz was conducted for 10 minutes, and an even mixturewas obtained. The mixture after pulverization was heated at 900° C. inair for 1 hour and crystals of a lithium excess solid solutionLi_(1.2)Mn_(0.56)Ni_(0.17)Co_(0.07)O₂ with an average particle diameterof 1 μm or less was obtained. 91% by mass of these crystal particles and4% by mass of the conductive carbon of Example 1 were mixed, and then 5%by mass of polyvinylidene fluoride and an adequate quantity of N-methylpyrrolidone were added and kneaded sufficiently so that slurry wasformed, and this slurry was coated on an aluminum foil, dried and thengiven a rolling treatment, and a positive electrode with an activematerial layer for a lithium ion secondary battery was obtained. Theelectrode density of the positive electrode was calculated from themeasured values of the volume and weight of the active material layer onthe aluminum foil in the positive electrode. The value of the electrodedensity was 3.15 g/cc. Further, using the positive electrode obtained, alithium ion secondary battery was manufactured in which 1M LiPF₆ in a1:1 ethylene carbonate/diethyl carbonate solution was used as anelectrolytic solution, and in which lithium was used as a counterelectrode. Charging/discharging characteristics of the lithium ionsecondary battery obtained were measured for a broad range of currentdensities.

Comparative Example 8

91% by mass of Li_(1.2)Mn_(0.56) Ni_(0.17) Co_(0.07)O₂ particles thatwere obtained in Example 9 and 4% by mass of acetylene black (primaryparticle diameter: 40 nm, specific surface area of micropores: 0 m²g⁻¹)were mixed, and then 5% by mass of polyvinylidene fluoride and anadequate quantity of N-methyl pyrrolidone were added and kneadedsufficiently so that slurry was formed, and this slurry was coated on analuminum foil, dried and then given a rolling treatment, and a positiveelectrode with an active material layer for a lithium ion secondarybattery was obtained. The electrode density of the positive electrodewas calculated from the measured values of the volume and weight of theactive material layer on the aluminum foil in the positive electrode.The value of the electrode density was 2.95 g/cc. Further, using thepositive electrode obtained, a lithium ion secondary battery wasmanufactured in which 1M LiPF₆ in a 1:1 ethylene carbonate/diethylcarbonate solution was used as an electrolytic solution, and in whichlithium was used as a counter electrode. Charging/dischargingcharacteristics of the lithium ion secondary battery obtained weremeasured for a broad range of current densities.

FIG. 10 is a graph that shows the relationship between the rate and thedischarge capacity per volume of the positive electrode active materiallayer in the lithium ion secondary batteries of Example 9 andComparative Example 8. In line with the result shown in FIG. 6, FIG. 10shows that the discharging capacity increases as the electrode densityincreases and almost the same rate characteristics are obtained. For thelithium ion secondary batteries of Example 9 and Comparative Example 8,charging/discharging was repeated within the range of 4.8 to 2.5 V underthe condition of 25° C. and the charging/discharging rate of 0.5 C. FIG.11 shows the result of the cycling characteristics obtained. As with theresult shown in FIG. 7, FIG. 11 shows that the secondary battery ofExample 9 has better cycle characteristics than the secondary battery ofComparative Example 8.

(iv) Change of Carbon Raw Material

Example 10

10 g of furnace black with pores (average primary particle diameter: 20nm, specific surface area of micropores: 1131 m²g⁻¹) was added to 300 mLof 60% nitric acid, an ultrasonic wave was irradiated for 10 minutesinto the fluid obtained, and the fluid was filtrated and the furnaceblack was retrieved. The retrieved furnace black was washed with waterthree times and then dried, so that oxidized furnace black was obtained.0.5 g of this oxidized furnace black, 1.98 g Fe(CH₃COO), 0.77 gLi(CH₃COO), 1.10 g C₆H₈O₇.H₂O, 1.32 g CH₃COOH, 1.31 g H₃PO₄, and 120 mLdistilled water were mixed. The compound fluid obtained was agitated for1 hour by a stirrer, and then the compound fluid was evaporated, driedand solidified at 100° C. in air, and a mixture was collected. Then, themixture obtained was introduced into a vibratory ball mill device andpulverization was conducted at 20 hz for 10 minutes. The powder afterpulverization was heated at 700° C. for 3 minutes in nitrogen, and acomplex in which LiFePO₄ was supported by the furnace black wasobtained.

1 g of the complex obtained was added to 100 mL of 30% hydrochloric acidsolution, and LiFePO₄ in the complex was dissolved while the fluidobtained was irradiated by an ultrasonic wave for 15 minutes, and thenthe remaining solid body was filtered, washed by water, and dried. Apart of the solid body after drying was heated to 900° C. in air andweight loss was measured by TG analysis. The process of dissolution ofLiFePO₄ by the aforementioned hydrochloric acid solution, filtration,water washing and drying was repeated until it was confirmed that weightloss was 100%, that is, no LiFePO₄ remained, so that conductive carbonthat did not contain any LiFePO₄ was obtained.

Then, 0.1 g of the conductive carbon obtained was added to 20 mL ofammonia solution with pH 11, and ultrasonic irradiation was applied for1 minute. The fluid obtained was left for 5 hours and a solid phase areawas precipitated. After the precipitation of the solid phase area, thesupernatant fluid was removed, the remaining part was dried, and theweight of the solid object after drying was measured. By subtracting theweight of the solid object after drying from the weight of the initialconductive carbon (0.1 g) and calculating the weight ratio of thesubtracted result against the initial weight of the conductive carbon(0.1 g), the contained amount of the “hydrophilic part” in theconductive carbon was evaluated. This conductive carbon contained 13% ofhydrophilic part. The contained amount of the hydrophilic part infurnace black with pores, which was used as a raw material, was only 2%.

94% by mass of commercially available LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂particles (average diameter: 5 μm), 2% by mass of conductive carbonobtained, and 2% by mass of acetylene black (primary particle diameter:40 nm, specific surface area of micropores: 0 m²g⁻¹) were mixed. FIG. 12shows an SEM image of the mixture obtained at a magnification of 50,000.The surface of the particles is partly covered with a paste-likematerial and their outline form is not clearly identifiable; thispaste-like material is the conductive carbon that is obtained byoxidizing the furnace black raw material, which spreads while coveringthe surface of the particles due to the pressure during mixing. Also, itcan be observed that fine furnace black with an average primary particlediameter 20 nm and acetylene black with an average primary particlediameter 40 nm are well dispersed. It is generally said that fineparticles easily aggregate, but by virtue of the conductive carbonobtained by oxidizing furnace black with pores, aggregation of fineparticles is effectively inhibited.

Then, 2% by mass of the total mass of polyvinylidene fluoride and anadequate quantity of N-methyl pyrrolidone were added to the mixtureobtained and kneaded sufficiently so that slurry was formed, and thisslurry was coated on an aluminum foil, dried and then given a rollingtreatment, and a positive electrode with an active material layer for alithium ion secondary battery was obtained. The electrode density of thepositive electrode was calculated from the measured values of the volumeand weight of the active material layer on the aluminum foil in thepositive electrode. The value of the electrode density was 3.80 g/cc.Further, using the positive electrode obtained, a lithium ion secondarybattery was manufactured in which a solution of 1M LiPF₆ in a 1:1ethylene carbonate/diethyl carbonate solution was used as anelectrolytic solution, and in which lithium was used as a counterelectrode. For the battery obtained, charging/dischargingcharacteristics were evaluated for a broad range of current densities.Also, charging/discharging was repeated within the range of 4.6 to 3.0 Vunder the condition of 60° C. and the charging/discharging rate of 0.5C.

Example 10 and Comparative Example 6 are different in terms of the kindof carbon used for a positive electrode, but otherwise the same. InExample 10, conductive carbon obtained from a furnace black raw materialwith pores and acetylene black were used, whereas in Comparative Example6, only acetylene black was used. The electrode density of the positiveelectrode in Comparative Example 6 was 3.40 g/cc, so the electrodedensity was significantly improved by using conductive carbon obtainedfrom a furnace black raw material with pores. Example 10 and Example 6are also different in terms of the kind of carbon used for a positiveelectrode, but otherwise the same. In Example 6, conductive carbon thatwas obtained from a Ketjen Black raw material and acetylene black wereused, whereas in Example 10, conductive carbon obtained from a furnaceblack raw material with pores and acetylene black were used. Theelectrode density of the positive electrode in Example 6 was 3.81 g/cc,so almost the same electrode density was obtained notwithstanding thedifference in the raw materials in the conductive carbon.

FIG. 13 shows the relationship between the rate and the dischargecapacity per volume of the positive electrode active material layer ofthe lithium ion secondary batteries in Example 10 and ComparativeExample 6, and FIG. 14 shows the result of the cycling characteristicsof the lithium ion secondary batteries in Example 10 and ComparativeExample 6. FIG. 13 shows that the discharge capacity increases as theelectrode density increases and almost the same rate characteristics areobtained. Also, comparison of the rate characteristics of the secondarybattery of Example 6 in FIG. 6 and the rate characteristics of thesecondary battery of Example 10 in FIG. 13 reveals that almost the samerate characteristics are obtained notwithstanding the difference in theraw materials in the conductive carbon used for the positive electrodes.FIG. 14 shows that the secondary battery of Example 10 has bettercharacteristics than the secondary battery of Comparative Example 6.Also, comparison of the cycling characteristics of the secondary batteryof Example 6 in FIG. 7 and the cycling characteristics of the secondarybattery of Example 10 in FIG. 14 reveals that almost the same ratecharacteristics can be obtained notwithstanding the difference in theraw materials in the conductive carbon used for the positive electrodes.

(4) Solubility of Active Material

As mentioned above, it is considered that the excellent cyclecharacteristics of the lithium ion secondary battery with a positiveelectrode that has an active material layer containing the conductivecarbon of the present invention is because almost all the surface of theparticles of the active material is covered with the paste-like carbonand this paste-like carbon inhibits the degradation of the activematerial. To confirm this, the solubility of the active material wasinvestigated.

Each of the conductive carbon in Example 1 and acetylene black was mixedwith LiFePO₄ particles with an average diameter of 0.22 μm, LiCoO₂particles with an average diameter of 0.26 μm, andLiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ with an average diameter of 0.32 μm at theratio by mass of 5:95, and then 5% by mass of the total mass ofpolyvinylidene fluoride and an adequate quantity of N-methyl pyrrolidonewere added and kneaded sufficiently so that slurry was formed, and thisslurry was coated on an aluminum foil, dried and then given a rollingtreatment, and an electrode was obtained. By using this electrode and anelectrolyte in which 1,000 ppm water was added to a solution of 1M LiPF₆in a 1:1 ethylene carbonate/diethyl carbonate solution, a coin-typebattery was manufactured. In this test, fine particles with a largespecific surface area were used in order to increase the area of theactive material that contacts the electrolytic solution. Also, 1000 ppmwater was added for the purpose of conducting an accelerated testbecause an active material dissolves more easily when there is morewater. This battery was left for 1 week at 60° C., then it wasdisintegrated, and then the electrolyte was collected and the amount ofmetal dissolved in the electrolyte was analyzed by using an ICP emissionanalysis device. Table 1 shows the result obtained.

TABLE 1 amount of dissolution of active material/% decrease ratio Mn FeCo Ni (against acetylene black) LiFePO₄ + acetylene black 1.52 LiFePO₄ +carbon in Example 1 0.93 −39% LiCoO₂ + acetylene black 13.1 LiCoO₂ +carbon in Example 1 6.05 −54% LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ + acetyleneblack 4.5 1.59 1.32 LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ + carbon in Example 11.9 1.35 0.93 −44%

As is evident from Table 1, the conductive carbon of Example 1remarkably inhibits the dissolution of the active material into theelectrolyte, compared with acetylene black. This is conceivably becausethe conductive carbon of Example 1, even if its active material is fineparticles with an average particle diameter of 0.22 to 0.32 μm,effectively inhibits the aggregation of these fine particles and coversalmost all the surface of the particles of the active material.

(5) Effect of Applying Pressure to Conductive Carbon

(i) SEM Observation

FIG. 15 shows SEM images of the coating films made by using theconductive carbon of Example 1 and the conductive carbon of ComparativeExample 4, in which each was dispersed in a dispersion medium, and adispersion obtained was coated on an aluminum foil and dried, and SEMimages of the coating films after a rolling treatment with the force of300 kN was given to the coating films. The coating film made with theconductive carbon of Comparative Example 4 did not exhibit a significantchange before and after the rolling treatment. However, from the SEMimages of the coating film made with the conductive carbon of Example 1,it was observed that the asperity of the surface was remarkablydecreased by the rolling treatment, the carbon particles were spread andthe grain boundaries were hardly discernable. Therefore, it was foundthat the characteristics of the carbon changed significantly due to theapplication of a strong oxidizing treatment.

(ii) Relationship Between Pressure and Density

For each of the conductive carbon of Example 1 and the conductive carbon(the carbon raw material) of Comparative Example 5, the conductivecarbon and polyvinylidene fluoride were added to an adequate quantity ofN-methyl pyrrolidone at the ratio of 70:30 and kneaded sufficiently sothat slurry was formed, and this slurry was coated on an aluminum foil,dried and then given a rolling treatment. FIG. 16 is a graph that showsthe result of investigating the influence of pressure by the rollingtreatment on carbon density; (A) shows the relationship between pressureand density and (B) shows the relationship between pressure and the rateof increase in density, respectively. As is evident from FIG. 16, theconductive carbon of Example 1 has a higher density, even beforepressure is applied, than the conductive carbon of Comparative Example5. Also, the conductive carbon of Example 1 is significantly moreaffected by pressure than the conductive carbon of Comparative Example 5and undergoes a large rate of increase in density with increasing ofpressure; when the pressure is 25 kPa, the rate of increase in densityexceeds 200%. Therefore, it is found that, if a rolling treatment isgiven under the condition of the same pressure, the conductive carbon ofExample 1 is more densely compressed than the conductive carbon ofComparative Example 5. From this it can be understood that, if anelectrode is produced with an electrode material containing theconductive carbon of Example 1, the conductive carbon of Example 1 isdensely compressed by a rolling treatment, the particles of an activematerial approach each other, and electrode density is improved.

(iii) Relationship Between Pressure and Conductivity

For each of the conductive carbon of Example 1 and the conductive carbon(the carbon raw material) of Comparative Example 5, the conductivecarbon and polytetrafluoroethylene were added to an adequate quantity ofN-methyl pyrrolidone at the ratio of 70:30 and kneaded sufficiently sothat slurry was formed, and this slurry was shaped in a sheet-likemanner, dried and then given a rolling treatment. FIG. 17 is a graphthat shows the result of investigating the influence of pressure by therolling treatment on carbon conductivity; (A) shows the relationshipbetween pressure and conductivity and (B) shows the relationship betweenpressure and the rate of increase in conductivity. As is evident fromFIG. 17, the conductive carbon of Example 1 has lower conductivity thanthe conductive carbon of Comparative Example 5, but the conductivity isincreased in accordance with the increase in pressure because density isincreased by the rolling treatment. When an electrode is produced withan electrode material containing the conductive carbon of Example 1, theconductive carbon of Example 1 spreads in a paste-like manner on thesurface of the particles of the active material and the contact area ofthe conductive carbon and the particles of the active materialincreases, and this conductive carbon that is spread in a paste-likemanner is densely compressed due to contact with the particles of theactive material and the conductivity of the conductive carbon isimproved.

(6) Mixing State of Conductive Carbon and Active Material

The following experiment was performed to confirm the mixing state of anactive material and carbon.

(i) Mixture of Fine Particles and Carbon

Each of the conductive carbon of Example 1 and acetylene black wasintroduced into a mortar with LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ fineparticles with an average particle diameter of 0.32 μm at the ratio bymass of 20:80 and dry blending was conducted. FIG. 18 shows SEM imagesat a magnification of 50,000. It was found that, when acetylene black isused as carbon, compared with the case where the conductive carbon ofExample 1 is used, fine particles aggregate even under the same mixingcondition. Therefore, it was found that the conductive carbon of Example1 effectively inhibits the aggregation of fine particles.

(ii) Mixture of Gross Particles and Carbon

Each of the conductive carbon of Example 1 and acetylene black wasintroduced into a mortar with LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ grossparticles with an average particle diameter of 5 μm at the ratio by massof 4:96 and dry blending was conducted. FIG. 19 shows SEM images at amagnification of 100,000. It was found that, when acetylene black isused as carbon, the gross particles and acetylene black existseparately, but when the conductive carbon of Example 1 is used, thegross particles are covered by a paste-like object, and the outline formof the gross particles is not clearly identifiable. This paste-likeobject is the conductive carbon of Example 1 that spreads while coveringthe surface of the gross particles due to the pressure of mixing. It isconsidered that, by a rolling treatment when an electrode is produced,the conductive carbon of Example 1 further spreads in a paste-likemanner and becomes dense while covering the surface of the particles ofthe active material, the particles of the active material approach eachother, and accordingly, the conductive carbon of Example 1 is pushed outinto the gaps formed between the adjacent particles of the activematerial and fills the gaps densely while covering the surface of theparticles of the active material, so that the amount of the activematerial per unit volume in the electrode is increased and the electrodedensity is increased.

INDUSTRIAL APPLICABILITY

By using the conductive carbon of the present invention, an electricstorage device with a high energy density can be obtained.

What is claimed is:
 1. A method of producing an electrode material foran electric storage device comprising oxidized conductive carbon andparticles of an electrode active material, comprising: acarbon-oxidizing step of obtaining the oxidized conductive carbon bygiving an oxidizing treatment to a carbon raw material so that theoxidized conductive carbon has a hydrophilic part in the amount of 10%mass or more of the entire oxidized conductive carbon; and a combiningstep of mixing the oxidized conductive carbon obtained in thecarbon-oxidizing step with the particles of an electrode active materialso that at least part of the oxidized conductive carbon is deformed intoa paste-like state by a pressure added to the oxidized conductive carbonduring the combining step and the deformed oxidized conductive carbon isattached to a surface of the particles of an electrode active material.2. The method of producing an electrode material for an electric storagedevice according to claim 1, wherein dry mixing is carried out in thecombining step.
 3. The method of producing an electrode material for anelectric storage device according to claim 1, wherein the oxidizingtreatment is carried out so that the oxidized conductive carbon has thehydrophilic part in the amount of 12% mass or more of the entireoxidized conductive carbon in the carbon-oxidizing step.
 4. The methodof producing an electrode material for an electric storage deviceaccording to claim 1, wherein the carbon raw material has 200 m²/g ormore of a specific surface area of micropores having a diameter of 2 nmor less as measured by the MP method.
 5. The method of producing anelectrode material for an electric storage device according to claim 4,wherein the carbon raw material is selected from a group consisting ofKetjen Black and furnace black with pores.
 6. The method of producing anelectrode material for an electric storage device according to claim 1,wherein the particles of an electrode active material have an averagediameter of more than 2 μm and not more than 25 μm.
 7. The method ofproducing an electrode material for an electric storage device accordingto claim 1, wherein the ratio by mass of the particles of an electrodeactive material and the oxidized conductive carbon is within the rangeof 95:5 to 99:1.
 8. The method of producing an electrode material for anelectric storage device according to claim 1, wherein the electrodematerial further comprises other conductive carbon having higherelectroconductivity than the oxidized conductive carbon, and the otherconductive carbon is mixed with the oxidized conductive carbon and theparticles of an electrode active material in the combining step so thatthe deformed oxidized conductive carbon is attached to a surface ofparticles of the other conductive carbon as well as the surface of theparticles of an electrode active material.
 9. The method of producing anelectrode material for an electric storage device according to claim 8,wherein dry mixing is carried out in the combining step.
 10. The methodof producing an electrode material for an electric storage deviceaccording to claim 8, wherein the ratio by mass of the particles of anelectrode active material and a total of the oxidized conductive carbonand the other conductive carbon is within the range of 95:5 to 99:1. 11.A method of producing an electrode for an electric storage device,comprising: a mounting step of forming an active material layer byapplying the electrode material produced by the method according toclaim 1 on a current collector for the electrode; and a pressurizingstep of adding pressure to the active material layer formed in themounting step so that the oxidized conductive carbon is further deformedinto the paste-like state.
 12. The method of producing an electrode foran electric storage device according to claim 11, wherein the activematerial layer formed in the mounting step is pressurized in thepressurizing step so that the deformed oxidized conductive carbon ispushed out not only into gaps that are formed between the adjacentparticles of an electrode active material but also into pores that existon the surface of the particles of an electrode active material andfills the gaps and the pores while covering the surface of the particlesof an electrode active material.
 13. The method of producing anelectrode for an electric storage device according to claim 12, whereinthe particles of an electrode active material have the gaps with a widthof 50 nm or less and/or the pores with a width of 50 nm or less.
 14. Themethod of producing an electrode for an electric storage deviceaccording to claim 11, wherein the active material layer in which thedeformed oxidized conductive carbon has pores with a diameter of 5 to 40nm is obtained in the pressurizing step.
 15. The method of producing anelectrode for an electric storage device according to claim 11, whereinthe active material layer in which 80% or more of the surface of theparticles of an electrode active material contact the deformed oxidizedconductive carbon is obtained in the pressurizing step.
 16. A method ofproducing an electrode for an electric storage device, comprising: amounting step of forming an active material layer by applying theelectrode material produced by the method according to claim 8 on acurrent collector for the electrode; and a pressurizing step of addingpressure to the active material layer formed in the mounting step sothat the oxidized conductive carbon is further deformed into thepaste-like state.
 17. The method of producing an electrode for anelectric storage device according to claim 16, wherein the activematerial layer formed in the mounting step is pressurized in thepressurizing step so that the deformed oxidized conductive carbon ispushed out not only into gaps that are formed between the adjacentparticles of an electrode active material but also into pores that existon the surface of the particles of an electrode active material andfills the gaps and the pores while covering the surface of the particlesof an electrode active material.
 18. The method of producing anelectrode for an electric storage device according to claim 17, whereinthe particles of an electrode active material have the gaps with a widthof 50 nm or less and/or the pores with a width of 50 nm or less.
 19. Themethod of producing an electrode for an electric storage deviceaccording to claim 16, wherein the active material layer in which thedeformed oxidized conductive carbon has pores with a diameter of 5 to 40nm is obtained in the pressurizing step.
 20. The method of producing anelectrode for an electric storage device according to claim 16, whereinthe active material layer in which 80% or more of the surface of theparticles of an electrode active material contact the deformed oxidizedconductive carbon is obtained in the pressurizing step.